The conventional tristimulus color gamut devised for color television broadcasting and adapted for conventional CRT displays used in computer monitors and related types of displays is based on the red, green, and blue light emitted from CRT phosphors. The CIE (Commission Internationale de l'Eclairage or “International Commission on Illumination”) Standard Colorimetric Observer, first drafted in 1931 and revised in years following, defines a color space in which the color gamut for a phosphor-based CRT display device can be represented. Any such color gamut within this color space is defined by the three primary colors that are emitted by a standard set of CRT phosphors. FIG. 1A shows a chromaticity diagram with a two-dimensional projection of the conventional broadcast television color gamut based on the CIE 1976 u′, v′ Metric Chromaticity Coordinates representation that is familiar to those skilled in the color display arts and is in conformance with International Telecommunications Union (ITU) specification ITU Rec. 709.
In this chromaticity representation, an outer curve, or spectrum locus 10 represents the range of pure colors, that is, colors of a single wavelength. The ends of the spectrum locus are connected by the line known as a purple boundary 11. The area bounded by spectrum locus 10 and purple boundary 11 contains the colors that can be perceived by the human visual system. An inner triangle 12 represents the conventional ITU Rec. 709 color gamut. Vertices 14r, 14g, 14b of triangle 12 are defined by the three primary CRT phosphor emission colors, red, green, and blue, respectively.
Referring to FIG. 1B, various regions and characteristics of the ITU Rec. 709 color gamut representation are indicated for reference. Neutral colors are approximately centered about a white point 20 within triangle 12. Constant hue lines 22 radiate outwards from white point 20. Colors on the same constant hue line 22 have the same hue, varying by saturation, which is proportional to the distance of the color coordinates from white point 20. For example, colors at coordinates 24 and 26 in FIG. 1B have the same hue; color 26 has increased saturation over color 24. Constant hue lines 22 are represented using dashed lines in FIG. 1B, which are substantially straight, exhibiting slight curvature when using this chromaticity representation.
As is readily apparent from FIGS. 1A and 1B and well known to those skilled in color reproduction, the ITU Rec. 709 color gamut represented by the area of triangle 12 is limited with respect to the range of color that could be represented by a display in the ideal case, represented by the full gamut of physically realizable colors contained within the region defined by the spectrum locus 10 and purple boundary 11. This is because CRT display phosphors, upon which the ITU Rec. 709 color gamut has been based, do not emit pure colors, that is, they do not emit light having a single wavelength. In terms of the graph of FIG. 1A, the limited gamut is represented by vertices 14r, 14g, 14b of triangle 12 lying well within the region bounded by spectrum locus 10 and purple boundary 11. Because of the relatively limited color gamut available using the ITU Rec. 709 encoding, many colors cannot be adequately represented and must therefore be approximated. This is particularly true for colors that are highly saturated.
The development of low-cost lasers at visible wavelengths now offers the promise of significantly increased color gamut in color display applications. This is because, unlike the CRT phosphors upon which the ITU Rec. 709 encoding is based, the laser emits light of nearly a single wavelength. Thus, in terms of the gamut representation in the chromaticity diagrams of FIGS. 1A and 1B, primary colors from laser sources lie on the periphery of spectrum locus 10, rather than well inside this curve, as is true for the CRT phosphor primaries of the ITU Rec. 709 gamut. In FIG. 1A, points 16r, 16g, and 16b represent the positions of laser color primaries within this spectrum locus, for one specific set of laser primaries. These and other such color vertices lying directly on spectrum locus 10 can provide a substantially greater possible color gamut obtainable by a display.
There has been some effort expended to take advantage of laser capabilities for color display. Methods and apparatus for adapting the color gamut capabilities of display systems that use laser primaries are described, for example, in the following:
Commonly assigned U.S. Pat. No. 6,802,613 entitled “Broad Gamut Color Display Apparatus Using an Electromechanical Grating Device” to Agostinelli et al., and No. 6,736,514 entitled “Imaging Apparatus for Increased Color Gamut Using Dual Spatial Light Modulators” to Horvath et al., disclose display apparatus using more than three lasers to expand the color gamut;
U.S. Pat. No. 6,774,953 entitled “Method and Apparatus for Color Warping” to Champion et al., discloses a method for using Look-Up Tables (LUTs) to adapt gamma-corrected R′G′B′ color data encoded for CRT display to an expanded color space afforded by a laser display. The Champion et al. '953 disclosure does not, however, describe how LUT values are derived.
While these and other patents describe how an expanded color gamut can be obtained and describe techniques for quick computation of transformed color data values suited to an alternate color gamut, however, the problems of accurate hue reproduction and preservation of near neutral colors have not been addressed. In terms of the graph of FIG. 1B, near-neutral colors are those within a relatively short distance from white point 20. Near-neutral colors include pastels and other low-saturation colors.
With the corresponding development of spatial light modulators that are ideally suited to handle laser illumination, such as the electromechanical conformal grating device disclosed in U.S. Pat. No. 6,307,663, entitled “Spatial Light Modulator with Conformal Grating Device” to Kowarz, for example, there is heightened interest in the possibility of expanding the relatively constrained ITU Rec. 709 color gamut and displaying colors that are more visually pleasing.
Two basic approaches have been followed for transforming the color gamut of the ITU Rec. 709 standard to that afforded by lasers. The first approach, as proposed in U.S. Pat. No. 5,440,352 entitled “Laser-Driven Television Projection System with Attendant Color Correction” to Deter et al., discloses a mapping of color data that simply adapts the gamut of a laser display to the conventional ITU Rec. 709 gamut, so that lasers simply replace the CRT phosphors. While this approach allows the use of laser illumination as a substitute for CRT display, however, it fails to take advantage of the broader color gamut afforded by lasers. In effect, the method proposed in the Deter et al. '352 disclosure simply performs a re-mapping of colors from the ITU Rec. 709 gamut, while also compensating for certain areas of the color gamut that may not be easily reached using lasers, but without an attempt to take advantage of the potentially broader color gamut afforded by a set of visible light lasers. While this approach allows the implementation of lasers for color display, color gamut expansion is not a goal of the Deter et al. '352 disclosure. Typically, a Look-Up Table (LUT) or 3×3 matrix is used to provide color transformation, mapping input ITU Rec. 709 values in one color gamut to the expanded output color gamut.
A second approach follows the solution of simply remapping a smaller color gamut to a larger one. In its most basic form, this approach simply applies the ITU Rec. 709 encoded data values directly to the broad gamut of the laser display, without applying any type of transformation to the data values. Unlike the re-mapping of the Deter et al. '352 disclosure, this second approach proposes expansion of the color gamut to take advantage of the pure wavelengths of laser emission. In conventional use, this basic approach has been found appropriate, for example, where one set of CRT phosphors provides an incremental increase in gamut over another set of CRT phosphors; this would correspond to slightly expanding the area of triangle 12 in FIG. 1A. For such a case, the increased saturation could provide a more appealing display of color and any subtle hue changes that might result may be imperceptible.
While this second approach would be suitable in moving from one phosphor set to an improved phosphor set, where there is incremental expansion of color gamut, this approach is not ideal for transformations between the ITU Rec. 709 phosphor primaries and laser primaries, where a substantial color gamut transformation is possible. Where larger chromatic increments are involved, color re-mapping or transformation from one color gamut to another is complicated by perceptual and psychophysical factors. Thus, experimentation has shown that a more pleasing or realistic color display of ITU Rec. 709 encoded color data is not necessarily achieved by simply expanding the color gamut, so that, for example, points 16r, 16g, and 16b on curve 10 now serve as the new primary colors, providing vertices for a broadened color gamut. Even though a significantly broader range of colors can now be displayed, simply remapping colors to a broadened color gamut does not necessarily provide a satisfactory result.
In transforming colors represented in a restricted-gamut encoding, such as ITU Rec. 709, to a gamut using laser primaries, the conventional techniques that worked well enough when handling subtle changes between different CRT phosphor sets have been shown to be less than satisfactory. With respect to FIG. 1B, near-neutral colors and flesh tones, for example, may no longer appear realistic when using conventional re-mapping techniques. Other undesirable hue changes are noticeable, particularly since laser display primary colors typically differ in hue from phosphor primary colors.
Thus, in spite of the promise of considerably improved color representation with lasers, the results obtained when applying conventional gamut expansion techniques have been surprisingly disappointing. Therefore, while it seems that an expanded color gamut should yield significant improvements in the appearance of a color display, true improvements have proven to be somewhat more elusive.
There is, then, a need for display apparatus and methods that take advantage of the broadened color gamut afforded by laser illumination, to provide a display that is more visually pleasing and is well-suited to the color perception of the viewer when used in conjunction with standard broadcast-encoded color-image data.